Batch MKV to WebM Conversion in Your Browser

Take heavy MKV captures from OBS and turn them into lean VP9 WebM ready for the web.

Batch convert MKV (H.264/H.265) to VP9 WebM
Container simplification: one video + one audio track
Typically smaller than the source at matched quality
Local browser processing, no upload, no account
Download individually or as a ZIP

Drop video files here or click to browse

Files queue into the current batch until you convert · MP4, MOV, MKV, WebM, M4V

Overview

OBS-to-CMS is the workflow this page exists for. OBS captures land as MKV (a flexible container that can carry several audio tracks, subtitle tracks, and chapter atoms), and most CMSes (or your own self-hosted <video> tag) want the opposite: one video track, one audio track, modern codec, lean container. WebM with VP9 fits that brief, with the caveat that the converter only carries the primary video and audio track through.

The video usually moves from H.264 inside MKV to VP9 inside WebM, which often produces a smaller file at matched perceived quality. The container itself also gets simpler, with no chapter atoms, no extra audio streams, and no embedded subtitles.

The conversion runs locally in your browser, so a stack of multi-hour OBS captures works the same as a few short clips. No upload, no server timeout.

Technical notes

Container simplification

MKV can carry many audio tracks, subtitles, and chapters. The WebM output keeps just the primary video and audio track, which is exactly what HTML <video> wants.

VP9 efficiency

Re-encoding from H.264 or H.265 MKV to VP9 WebM often produces smaller files at matched perceived quality, with the largest gains on talking-head and screen content. Exact ratios vary with content.

Browser compatibility

VP9 WebM plays in Chrome, Edge, Firefox, and Safari 14.1+. Pair it with an MP4 fallback in your <video> tag for older Safari support.

Good to know

Subtitles and extra tracks dropped

MKV files can hold multiple audio tracks, subtitles, and chapters. The WebM output keeps only the primary video and audio track. Ship subtitles as a separate WebVTT file in your <video> element.

VP9 encode is CPU-bound

VP9 software encoding is slower than typical H.264 hardware encoding. Long OBS sessions take meaningful time to encode. Start the batch and let it run.

How to convert MKV to WebM in bulk

Convert a folder of MKV files to WebM in your browser without uploading anything.

Add your MKV files

Drag and drop one or many MKV files into the queue, or click to pick them.

Confirm the output preset

Output is preset to WebM. For web embedding, drop the width to 1280 or 1920 and use the balanced quality preset.

Start the batch

Click Convert files. Each file is processed locally in your browser.

Download WebMs

Download converted WebM files individually, or grab the whole batch as a ZIP.

Frequently Asked Questions

Are my MKV files uploaded anywhere?

No. The conversion runs in your browser via WebCodecs and your files stay on your device.

What happens to subtitle tracks?

They are not carried into the WebM output. If you need subtitles for the web, ship them as a separate WebVTT track in your <video> element.

Will the file actually be smaller?

In most cases, yes. VP9 is more efficient than the H.264 typically found inside OBS-captured MKVs, and the WebM container has less overhead than MKV.

Why not stick with MKV?

Browsers do not play MKV natively in <video> tags. WebM is the closest spiritual cousin that plays everywhere modern.

Which browsers does this work in?

Conversion uses the WebCodecs API. Fully supported: desktop Chrome 94+, Edge 94+, Opera 80+, Firefox 130+, and Safari 26+. Safari 16.4 to 25 work for most files but some may fail because their WebCodecs implementation is partial. Browsers without WebCodecs (Chrome/Edge <94, Firefox <130, Safari ≤16.3, Internet Explorer) cannot run the converter at all. Mobile browser support is improving but inconsistent, so a desktop or laptop is recommended for batch work.